The present invention relates to methods, that can be performed automatically in a chip tester, which prevent a chip from being thermally destroyed by a defective pressed joint between the chip and a temperature regulating component within the chip tester. As used herein, the term xe2x80x9cchipxe2x80x9d means any of the following items: 1) an integrated circuit that is encapsulated in a package, such as a plastic or ceramic package; 2) an integrated circuit by itself without an encapsulating package; and 3) the integrated circuit of items 1) or 2) which is mounted on a substrate.
In the prior art, the structure of one chip tester is disclosed in U.S. Pat. No. 6,325,662. All of the teachings of that patent are herein incorporated by reference; however, FIGS. 2 and 2A in the patent show a portion of the chip tester that is most relevant to the present invention. Those figures are reproduced herein as FIGS. 1 and 2, and they are labeled prior art.
The above prior art chip tester includes a frame that has four vertical members, two of which are shown herein in FIGS. 1 and 2 as items 11e and 11f. These members support multiple sets of: a chip holding subassembly, a power converter subassembly, a temperature regulating subassembly, and a pressing mechanism.
Each chip holding subassembly includes components 12a-12d. From one to fourteen of these chip holding subassemblies are in the frame. Component 12a is a printed circuit board which has one face 12a-1 and an opposite face 12a-2. Attached to face 12a-1 are N sockets 12b, each of which holds one IC chip 12c that is to be tested. Here, N is any desired number, such as sixteen or thirty, for example. Attached to face 12a-2 are N sets of electrical contacts 12d, and each set carries all of the electrical power and all signals for one of the chips 12c. Each socket 12b is connected to one set of contacts 12d by microscopic conductors (not shown) that pass thru the printed circuit board 12a. 
Each power converter subassembly includes components 13a-13c. A separate power converter subassembly is supported by the frame above each chip holding subassembly. Component 13ais a printed circuit board which has one face 13a-1 and an opposite face 13a-2. Attached to face 13a-1 are N sets of electrical contacts 13b, each of which mates with one set of the contacts 12d on the chip holding subassembly. Attached to face 13a-2 are N DC-DC power converters 13c. Each power converter 13c supplies power to one set of the contacts 13b, and it is connected to those contacts by microscopic conductors (not shown) that pass through the printed circuit board 13a. 
Each temperature regulating subassembly includes components 14a-14d. A separate temperature regulating subassembly is in the frame below each chip holding assembly 12. Component 14a is a flat rigid plate which has one face 14a-1 and an opposite face 14a-2. Attached to face 14a-2 are N springy components 14b, and each springy component 14b holds one temperature regulating component 14c such that it is aligned with one chip 12c in the chip holding assembly 12.
The temperature regulating component 14c can be of a type which removes heat from the chips 12c by conduction, such as a heat sink; or it can be of a type which adds heat to the chips 12c by conduction, such as an electric resistive heater; or it can be a combination of both types. Several stops 14d are attached to the face 14a-2, and they are aligned with the spaces between the sockets 12b in the chip holding assembly. These stops 14d limit the force with which the temperature regulating components 14c can be pressed against the chips 12c. 
Each pressing mechanism includes components 15a-15g. Component 15a is a rail which is rigidly attached to the frame columns 11e and 11f. This rail 15a lies below the temperature regulating subassembly and is parallel to face 14a-1 of the plate 14a. Components 15b and 15c are a pair of arms that are coupled together with a pivotal joint 15d which presses against face 14a-1 of the plate 14a. The arms 15b and 15c also have slidable joints 15e and 15f which slide on the rail 15a. Component 15g is a spring which is coupled between the slidable joint 15f and the frame. All of the components 15b-15g are duplicated in the pressing mechanism as shown in FIG. 1.
In operation, an actuator slides the arms 15b on the rail 15a to either an xe2x80x9copenxe2x80x9d position or a xe2x80x9cclosedxe2x80x9d position. When the arms 15b are in the open position, the angle xe2x80x9cAxe2x80x9d between the arms 15b and 15c is large, and so the pivotal joints 15d have moved down. Consequently, each chip holding subassembly is spaced apart from its corresponding power converter subassembly and corresponding temperature regulating subassembly, as is shown in FIG. 1.
Conversely, when the arms 15b are in the closed position, the angle xe2x80x9cAxe2x80x9d between the arms 15b and 15c is small, and so the pivotal joints 15d have moved up. Consequently, each chip holding subassembly is pressed against its corresponding power converter subassembly and corresponding temperature regulating subassembly, as is shown in FIG. 2.
To test a set of chips with the tester of FIGS. 1 and 2, the following sequence of steps conventionally is performed. First, while the arms 15b are in the open position, each chip holding subassembly is placed in the tester between its corresponding power converter subassembly and corresponding temperature regulating subassembly. Next, the arms 15b are moved to the closed position, and in that position electrical power and test signals are sent to all of chips 12c. While this occurs, the temperature of the chips 12c is regulated by the temperature regulating components 14c. Then, after all of the test signals have been sent to the chips 12c, the electrical power to chips is turned off, the arms 15b are moved back to the open position, and each chip holding subassembly is removed from the tester.
However, a major drawback with the above sequence of steps is that when the arms 15b are in the closed position, a defect may be present in one or more of the pressed joints that occur between the chips 12c and the corresponding temperature regulating components 14c. Due to such a defect, the thermal resistance through the pressed joint can be so large that the temperature regulating component 14c is not able to prevent the chip 12c from overheating when electrical power is applied to chip.
One particular cause for a pressed joint being defective is that a chip 12c has been improperly inserted in its socket 12b. Another cause is that the surface of a temperature regulating component 14c which contacts a chip 12c has been oxidized by extended use, and thereby became too resistant. Still another cause is that a film of thermally resistant debris has been accidentally deposited on the surface of a chip 12c or the surface of a temperature regulating component 14c that gets pressed together.
The above problem is most serious for the latest state-of-the-art chips which dissipate extremely high levels of electrical power. Some of the latest chips dissipate over two-hundred watts of power, and at that power level a chip will rapidly destroy itself if it is not properly cooled. Starting at about 150 degrees centigrade, various materials that make up the chip can either improperly diffuse, or soften, or melt.
Accordingly, a primary object of the present invention is to overcome the above problem.
The present invention is a method of preventing the thermal destruction of an integrated circuit chip in a tester that includes a temperature regulating component for contacting said chip through a pressed thermal joint, which could be defective. This method begins with the step of maintaining the chip at an ambient temperature while forcing the temperature regulating component to a different temperature, when the chip and the temperature regulating component are spaced apart in the tester. Then, the chip at its ambient temperature and the temperature regulating component at its different temperature are pressed together. Next, a temperature change is sensed in either the chip or the temperature regulating component, during a time interval that begins when the temperature regulating component and the chip initially press together. Thereafter, electrical power is applied to the chip in the tester only if the temperature change, which is sensed by the sensing stop, meets a predetermined criteria. This method is based on certain thermodynamic principles which are explained in the Detailed Description.
In one particular version of the above method, the sensing step is performed by an electronic sensor in the temperature regulating component; and, electrical power is applied to the chip only if the temperature change, which is sensed by the sensing step, is larger than a preset limit. In one other particular version, the sensing step is performed by an electronic sensor in the chip; and, electrical power is applied to the chip only if the temperature change, which is sensed by the sensing step, is larger than a preset limit.
In another particular version, the step of forcing the temperature regulating component to a different temperature is performed by including a hollow heatsink in the temperature regulating component and passing a fluid at a constant temperature through the hollow heatsink. In still another particular version, the step of forcing the temperature regulating component to a different temperature is performed by including an electric heater in the temperature regulating component and passing a current with a constant magnitude through the electric heater.